Silverlight is a browser plugin developed by Microsoft that allows creation of rich interactive applications and media experiences using HTML, CSS, and .NET languages like C# and VB. It provides a runtime for browser-based rich internet applications that can run across browsers on Windows, Mac OSX, and Linux. Silverlight applications are built with XAML markup and code files and use WPF and .NET frameworks to provide capabilities like media streaming, flexible programming models, and security.